Most dryers for particulate material, such as grain, make relatively inefficient use of the energy which is expended in the drying process. The commonest dryers of this type burn a fuel such as liquid propane and utilize the heated air and products of combustion as the sole source of heat for drying the grain, except to the extent that metal parts of the drying chamber may also become heated by the air and combustion products.
A patent which discloses a different heat source is McOmber U.S. Pat. No. 2,777,212, which discloses a grain drying system mounted upon a combine in which the heat from an internal combustion engine for driving the combine is utilized to heat the air for drying grain.
Either of the above types of dryers is relatively inefficient because they continually introduce unheated air from the ambient atmosphere, heat it, pass it through the grain, and then discharge the moisture laden air to the ambient atmosphere.
Closed circuit drying systems for laundry dryers are disclosed in Whitesel U.S. Pat. No. 2,660,806 and Shewmon U.S. Pat. No. 2,676,418. They utilize a thermal pump system with a heat exchanger in a duct upstream from a tumbling drum and the heat pump condenser in the duct downstream from the drum. The heat pump compressor delivers compressed fluid through the heat exchanger to the condenser. A similar arrangement for removing cleaning solvent from textiles is disclosed in Freze U.S. Pat. No. 3,831,294, except that there is an air heating coil upstream from the drum and an air cooling coil downstream from the drum; but without utilizing a thermal pump system.
The foregoing systems make relatively inefficient use of the energy which heats the fluid for the heat exchanger because only the circulating air, rather than the material to be dried, comes into contact with the heat exchanger.